Typically, communications between various terminals on a network are transmitted through dedicated medium such as coaxial cables, twisted pair wires, and fiber optic cables. These media can sometimes be expensive as they have to meet stringent transmission criteria. In addition, significant costs are incurred for physically routing the medium to each of the various terminals. Hence, it would be beneficial if the terminals could take advantage of an already existing medium for communication purposes.
One such medium is power distribution lines such as those found in virtually all homes, offices, and factories. In many cases these power lines distribute 120 volt AC (alternating current) to wall sockets, thereby supplying power to various devices such as appliances, computers, lights, etc. Because power lines are designed primarily for transmitting power, they are not ideally suited for communications. One major problem is that noise on power lines can be quite high due to electrical interference emanating from the very devices being powered.
In the prior art, spread spectrum communications techniques have been utilized in noisy environments, including power lines. Spread spectrum involves transmitting communications signals over a frequency spectrum that is purposely made broad with respect to the information bandwidth. The signal is subsequently despread and decoded at the receiving end. In this manner, spread spectrum receivers have the highly desirable ability to enhance the expected signal while suppressing the effects of all other inputs.
In order to distinguish data from noise and to maximize the output signal to noise ratio (SNR), a correlation function is typically implemented at the receive end. The correlation function compares signals on the power line against an expected signal. Ideally, noise does not match the expected waveform, and valid signals result in perfect matches. Unfortunately noise does, by its random nature, occasionally exhibit a measure of correlation to the expected signal (e.g., sometimes as much as 50% correlation). Equally unfortunate is the fact that signals propagating through power distribution lines are frequently attenuated and distorted. When received in conjunction with typical noise, these valid signals frequently exhibit low correlations (e.g., sometimes as low as 30%). Thus, noise might be misinterpreted as valid data and vice versa.
In response, spread spectrum receivers, typically employ a correlation threshold function which either by itself or in conjunction with other criteria, are used in an attempt to reliably distinguish valid signals from noise. The output of this threshold function process is a carrier detect signal which indicates the likely presence of valid data signals. Even so, false carrier detects can occur whenever noise has a correlation value exceeding the threshold. A false carrier detect can trigger the receiver to erroneously synchronize on the noise and thereby miss a real data packet. Another problem is that when a carrier is detected on a CSMA network (Carrier Sense Multiple Access--a network access method wherein multiple transceivers share a medium), a channel busy indicator is typically generated to keep the transmitter from transmitting and causing a collision. Consequently, false carrier detections reduce the overall network throughput.
The goal then is to fix a correlation threshold such that random noise does not trigger a carrier detect indication, while valid receive signals of less than ideal correlation do trigger carrier detect indications. However, there is a dilemma in choosing a value for setting the threshold. On the one hand, the threshold should be set at a low level in order to detect weak and/or distorted signals which are valid and have relatively low correlations. On the other hand, setting the threshold at too low a level risks having random noise breaking the threshold too frequently. Fixing a threshold is thus a compromise between either setting the threshold low enough to pick up signals having weak correlations but also falsely picking up noise or, conversely, setting the threshold high enough so that noise is not picked up but receive sensitivity is degraded.
Further complicating matters is the fact that the optimum threshold can vary, depending on the network environment. For example, one receiver may be operating in an environment having noise with a great degree of correlation, yet valid signals are strong and clean. The correlation threshold should be set relatively high in such situations. Conversely, for environments where the background noise has a low degree of correlation and signals are weak and distorted, the threshold should be set relatively low. Moreover, the network environment can change over time.
Therefore, what is needed is an apparatus and method for selecting an optimum threshold. It would also be highly preferable for the apparatus and method to be adaptive to its operating environment without adding a great deal of complexity. An adaptive threshold should increase the threshold level when noise having a relatively high degree of correlation is present. Although it is apparent which parameter needs to be adapted (i.e.,the carrier detect threshold), it is rather difficult to determine exactly how that parameter is to be adapted. In particular, it is difficult to determine what kind of information to use as a basis for setting the threshold.